Ca2+ sensitivity is a major factor that controls the contractile state of the heart. In a litany of cardiomyopathies (inherited and acquired), imbalances in Ca2+ sensitivity are commonly observed. We hypothesize that it is possible to specifically tune myocyte Ca2+ sensitivity and enhance or even curtail cardiac contraction. There are many ways Ca2+ sensitivity can be altered, but we suggest Ca2+ sensitivity can be simply controlled and strongly influenced by engineering the Ca2+ dependent switch in cardiac muscle, troponin C (TnC). We have designed TnCs with a wide range of Ca2+ binding properties. Through the use of adeno associated viruses we have introduced these designer proteins into the in vivo murine heart. Our exciting preliminary data demonstrates that expression of a Ca2+ desensitized TnC decreases contraction and leads to a rapid onset of a lethal dilated cardiomyopathy. On the other hand, increasing Ca2+ binding to TnC increases cardiac contraction without signs of disease, arrhythmia or diastolic dysfunction. This could be of major significance considering the contractility of cardiac muscle can be drastically compromised by many forms of heart disease, including myocardial infarction (MI) and pressure overload. Remarkably, our preliminary data also demonstrates that a Ca2+ sensitized TnC increases in vivo cardiac contractility after a MI or transverse aortic constriction (TAC), without compromising relaxation/diastolic function. The purpose of this proposal is multifold and will: 1) determine whether TnC can be rationally engineered to tune in vivo cardiac contraction; 2) define the role TnC's Ca2+ sensitivity plays in cardiac muscle mechanics in vivo; and 3) test whether altering the Ca2+ sensitivity of the heart by TnC can be used as a novel therapeutic approach to combat cardiac diseases with vastly different etiologies in mice. Aim 1. Can cardiac muscle contraction be tuned by adjusting the Ca2+ binding properties of TnC? Contractile function will be measured from myocyte shortening, to force in trabecula and ultimately in vivo using echocardiography and pressure-volume analysis. This battery of examinations will thoroughly test if cardiac muscle contraction can be controlled from the myocyte to in vivo by engineered TnC. Aim 2. Determine if altered Ca2+ binding to TnC can improve contractile function in acquired and inherited cardiomyopathies that have divergent etiologies. Although of vastly different etiology MI, TAC and dilated cardiomyopathy compromise the contractility of the heart. Contrarily, restrictive cardiomyopathy makes the heart hypercontractile. We will test if specifically engineered TnCs can be used to combat these diverse cardiac diseases and improve the in vivo performance of the heart. The completion of this proposal will clearly define the role Ca2+ binding to TnC plays in cardiac contraction and lead to novel translational strategies to help combat vastly different cardiac diseases.